A battery for supplying power to an electric motor is installed in a vehicle such as an EV (Electric Vehicle) or an HEV (Hybrid Electrical Vehicle). The battery includes plural series-connected storage cells, and a secondary cell such as a nickel-metal hydride or lithium-ion secondary cell is used as the storage cell. In order to use such secondary cells safely, the remaining capacity or state of charge (SOC) of each storage cell needs to be monitored at all times so as to implement a control for prevention of overcharge and overdischarge.
For example, an SOC may be calculated based on current integration. In this method, a charge current and a discharge current of a storage cell are integrated every predetermined time period to calculate an integrated charge quantity and an integrated discharge quantity. Then, the integrated charge quantity and the integrated discharge quantity are added to or subtracted from an initial SOC or an SOC immediately before the start of charge or discharge, thereby calculating an SOC of the storage cell. Hereinafter, thus-calculated SOC is referred to as an “integrated SOC.”
While the accuracy of the above-mentioned integrated SOC is high, in the battery installed in the vehicle, charge and discharge are repeated in the storage cells within a predetermined range of SOC, and the storage cells are used for a long time period. Therefore, errors may be accumulated in the integrated SOC. In addition, in calculating an integrated charge quantity and an integrated discharge quantity, for example, measuring errors of a current detector may be accumulated to increase the error of the integrated SOC. Further, since a reduction in design capacity of each storage cell due to self-discharge thereof as a result of the vehicle being left unused for a long time period is not integrated, the error of the integrated SOC may be further increased.
Alternatively, for example, an SOC may be calculated based on a terminal voltage of the battery cell which is being charged or discharged. There exists a relationship between an SOC and an open circuit voltage (OCV) of a storage cell, as exemplified in FIG. 18. In FIG. 18, an upper limit SOC and a lower limit SOC denote, respectively, an upper end value and a lower end value of a predetermined range of SOC where charge and discharge of a storage cell are repeated. In the storage cell having the characteristics shown in FIG. 18, the OCV varies largely at the upper limit SOC and the lower limit SOC but varies very little within the range therebetween.
Thus, when a SOC of the storage cell having the characteristics shown in FIG. 18 is calculated, while an SOC around an upper limit SOC and a lower limit SOC can be obtained with good accuracy, an SOC therebetween cannot be obtained with good accuracy. In addition, as described above, in the battery installed in the vehicle, since charge and discharge are repeated within the predetermined range of SOC in the constituent storage cells, the influence of a diffuse resistor becomes large, and the OCV of the battery cell varies largely. As a result, in many cases, the accuracy of the SOC calculated based on the terminal voltage becomes low.
While the above-mentioned two methods respectively have advantages and disadvantages, an accurate SOC may be obtained by combining these methods to complement the disadvantage of the other. In a remaining capacity or state-of-charge (SOC) detecting system in Patent Literature 1, a data substitution is performed on an integrated SOC by using a predetermined upper limit in accordance with a terminal voltage of a battery pack, and a difference between an integrated SOC which is calculated after the data substitution and a corrected SOC which is obtained by correcting the integrated SOC is corrected so as to increase as an integrated value of charge and discharge currents increases. Thus, even when a deviation between an integrated SOC and an actual SOC increases as an integrated value of charge and discharge currents increases, the accuracy of approximation of a corrected SOC to the actual SOC can be increased.